FIG. 1 is a diagram of an interlace structure according to a related art. Referring to FIG. 1, a 1xEV-DO broadcast system according to a related art designates a specific interlace as a broadcast slot on an interlace structure and enables broadcasting information to be transmitted using the slot.
Meanwhile, if orthogonal frequency division multiplexing (hereinafter abbreviated OFDM) is applied to the specific interlace, transmission capacity can be raised higher than that of the conventional code division multiple access (hereinafter abbreviated CDMA).
Due to the characteristics of the broadcast system, all base stations transmit the same signals, respectively. In case of using OFDM and a cyclic prefix having a suitable length, it is advantageous in that multi-path signals received within a corresponding delay duration interval defined by the cyclic prefix do not degrade the reception performance of a mobile station.
Namely, if a cyclic prefix enough to overcome a delay spread on a cell boundary is used, mobile stations around the cell boundary may regard a signal transmitted from a different base station as a multi-path signal only. Hence, a broadcast quality can be provided to the mobile stations on the cell boundary.
A currently proposed OFDM broadcast system is to transmit OFDM symbols carried by a data part of the broadcast slot shown in FIG. 1. By adopting the OFDM transmission system, a high broadcast data rate can be enabled. For example of the broadcast by OFDM, six data rates of 1.8 Mbps˜409.6 kbps, as shown in Table 1, can be supported by the combination of two payload sized (3072, 2048) and three slot lengths (1-slot, 2-slot, 3-slot).
TABLE 11st data rate set2nd data rate setData rateNumberData rateNumber(kbps)of slot(kbps)of slot1843.211228.81921.62614.42614.43409.63
Four data intervals, as shown in FIG. 1, exist in one interlace slot interval. Each of the data intervals includes 400 chips. If an OFDM block includes the 400 chips, two hundred forty 16-QAM (quadrature amplitude modulation) data symbols are transmitted via one OFDM block. Hence, 960 (240*4) encoded symbols are carried in one OFDM block.
FIG. 2 is a block diagram of a transmission chain for an OFDM broadcast system according to a related art. Referring to FIG. 2, data inputted to a turbo encoder 21 is encoded at ⅕ coding rate. The encoded data is scrambled by a scrambler 22.
The scrambled data goes through channel interleaving and 16-QAM processes 23 and 24 so that two hundred forty 16-QAM modulated data symbols are transmitted via one OFDM block. And, twenty guard tones are inserted in the OFDM block through cyclic shift-reordering and truncation processes 25 and 26 to prevent an aliasing effect. A pilot tone is then inserted, because it is necessary to estimate amplitude and phase information of a channel which varies according to time and subcarriers, for using the 16-QAM system.
For instance, one pilot tone can be inserted in each four data tones. In this case, total sixty-four pilot tones are inserted in one OFDM block. Where, the pilot tone inserted for twenty guard tone intervals replaces the guard tone. Hence, total three hundred twenty tones (=240 data tones+64 pilot tones+20 guard tones−4) are obtained. For this, an OFDM signal is generated using 320-dimensional IFFT (Inverse Fast Fourier Transform).
And, the remaining eighty chip intervals are used as a cyclic prefix. The cyclic prefix is inserted to remove neighbor symbol interference and neighbor subcarrier channel interference. According to a length of the intervals, a maximum delay spread value to keep the OFDM transmission may be decided.
Table 2 shows an example of a symbol configuration of an OFDM block according to a related art.
TABLE 2Cyclic prefix length80Number of Pilot tone64Number of Data tone240Number of Guard tone20Total number of tones in one OFDM block320
The currently proposed 1xEV-DO OFDM broadcast system adopts 16-QAM as a modulation method. As the 16-QAM transmits data using phase information and amplitude information together, its performance is greatly affected by phase and size variations of a channel. Hence, for the demodulation from the 16-QAM, the phase and size variations of the channel need to be estimated.
In the related art, pilot tones are inserted with each uniform interval for the channel estimation. In particular, one pilot tone is inserted each four data tones. However, additional power or bandwidth caused by the insertion of the pilot tones degrades the advantages in using the coherent 16-QAM.
Besides, due to the characteristics of OFDM, channel estimation according to time needs to be carried out for each three hundred twenty subcarriers. Accordingly, the channel estimation has to be carried out by considering frequency and time axes together. Hence, complexity pf a mobile station is raised.